Vegetable production is an important sector of the organic food industry in the Pacific Northwest USA. Significant acres of organic vegetable crops, particularly processing green pea and sweet corn, are grown in the semi-arid Columbia Basin of central Washington and north central Oregon, where Pythium damping-off can cause losses in early spring when cool and wet soil conditions are conducive to the disease. In this study, 37 certified organic fields were surveyed in the Columbia Basin for Pythium species, from which 305 isolates were baited and identified to 19 species. Pathogenicity tests of isolates of each species on pea in cool and wet soil conditions revealed isolates of 9 species were pathogenic, with differences in virulence among species and among isolates within species. Pythium ultimum (24.6% of the 305 isolates), P. irregulare group 1 (15.1%), and P. abappressorium (4.9%) were the most prevalent pathogenic species. Real-time PCR assays detected P. ultimumin 100% of the 37 fields compared to 78% for P. abappressorium, and 57% for P. irregulare group 1.
Given the lack of highly effective seed treatments for damping-off control in organic production, organic seed and drench treatments were evaluated in five pea field trials in the Columbia Basin, and two pea trials plus one sweet corn trial in maritime western Washington in 2011-12. Nordox seed treatment and seed priming demonstrated the greatest potential for damping-off control. Pea seed priming with 16 h of seed soaking + 10 h of air drying optimized priming for rapid emergence. In two field trials in 2012, combining Nordox seed treatment with seed priming using biochar as an alternative to air-drying, was promising for damping-off control.
Seed exudates produced during germination are measured using electrical conductivity (EC). The EC levels of 17 pea seed lots (six cultivars) significantly affected emergence and susceptibility to Pythium damping-off. A negative linear relationship was demonstrated between EC level and emergence, with a stronger regression at higher inoculum levels of P. ultimum. Measuring the EC of pea seed lots, and quantifying Pythium inoculum in soil using real-time PCR assays may enable organic pea growers to assess accurately the risk of damping-off.
|Advisor:||Toit, Lindsey J. du|
|Commitee:||Miles, Carol, Paulitz, Tim, Porter, Lyndon|
|School:||Washington State University|
|School Location:||United States -- Washington|
|Source:||DAI-B 75/02(E), Dissertation Abstracts International|
|Keywords:||Damping-off, Organic pea, Organic seed treatment, Organic vegetable production, Pythium spp., Seed exudation|
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